Aluminum base alloy bearing



ALUMINUM BASE ALLOY BEARING Alfred W. Schluchter, Dearborn, Mich.,assignor to General Motors Corporation, Detroit, Mich, a corporation ofDelaware No Drawing. Application December 27, 1952, Serial No. 328,266

7 Claims. (Cl. 75148) This invention relates to an aluminum base alloyand particularly to an improved alloy of this type which is especiallysuitable for use as a bearing material in the as-cast condition.

Aluminum and most of its alloys are generally quite unsuitable for usein bearings for ferrous metal machine parts because the aluminum tendsto adhere to, or combine with, the ferrous metal, thereby causingscoring or seizing. I have found, however, that by suitable combinationof alloying constituents, this difticulty can be overcome and a bearingalloy produced having not only anti-friction properties but othercharacteristics especially suitable in a bearing material.

Many aluminum base alloys, such as the type disclosed in Patent No.2,238,399, which issued April 15, 1941 in the name of Alfred W.Schluchter, are satisfactory bearing materials in most respects.However, most of these alloys do not possess sufficient hardness,especially in the as-cast state, to enable them to be satisfactorilyused for many purposes. In the past, many metals have been added toaluminum to improve its hardness, but most of these elements, such asiron, also impair the frictional properties of aluminum and aluminumbase alloys. Likewise, many of the alloys thus produced cannot besatisfactorily used in the as-cast condition.

Accordingly, a principal object of the present invention is to providean inexpensive aluminum base bearing alloy which possesses satisfactoryhardness and high score resistance. A further object of this inventionis to provide an aluminum base alloy which has highly desirablefrictional properties when used as a bearing in the as-cast conditionand which also may be heat treated to further increase its hardness.

In accordance with my invention, therefore, the foregoing and otherobjects and advantages are attained to a particularly high degree in analuminum base alloy containing minor proportions of silicon, bismuth andchromium. Inasmuch as an alloy of this composition is a stronger metalthan the aluminum alloys generally heretofore used for bearing purposes,solid bearings may be formed from it and no backing of steel or similarmetals is necessary. If desired, a bearing formed from my alloy may beadvantageously provided with a thin overlay of lead or a lead basealloy. Examples of these overlays include the lead-tin and lead-indiumalloys which are used for this purpose and in which lead is the majorconstituent.

The alloy formed in accordance with the present invention ischaracterized by greater hardness and a correspondingly longer fatiguelife than related aluminum base alloys heretofore used. As a result ofthis hardness, solid bearings made from this alloy retain their originalshapes better than many of the bearings which up to the present timehave been formed of softer alloys. The former do not take a set attemperatures to which they are normally subjected, and they undergo anegligible amount of shrinkage after extensive use. This type of niteStates Patent inexpensive bearing alloy is found to be particularlyvaluable for low speed applications where it may be used in as as-castcondition. Hence it may be satisfactorily employed as a cast bell endbearing in fractional horsepower motors.

Accordingly, I have found that satisfactory bearing properties areobtained with an alloy comprising, by weight, approximately 0.2% to 10%silicon, 0.1% to 5% bismuth, 0.1% to 0.8% chromium, and the balancesubstantially all aluminum. Various incidental impurities may beincluded in this alloy in the usual small amounts without anysubstantial detrimental effects. Thus the term aluminum, as used herein,embraces the usual impurities which are found in aluminum ingots ofcommercial grade or which are introduced during the handling operationsincident to ordinary melting practice. For example, iron, which togetherwith silicon is found in commercial aluminum, may be present in amountsnot greater than approximately 0.5% without causing any harmful results.For optimum results I have found that an alloy should be used whichconsists essentially of ap proximately 2% to 5% silicon, 0.3% to 2%bismuth, 0.2% to 0.35% chromium, and the balance substantially allaluminum.

Under severe test conditions, alloys having the above compositions showexcellent anti-friction properties so that bearings formed of this alloydo not score or gall when in contact with a rotating steel shaft, andneither the shaft nor the bearing show an appreciable amount of wearafter long and severe use. I have also found that the resistance of thisalloy to cracking or crumbling is extraordinarily high.

The inclusion of silicon in my aluminum base bearing alloy improves itsfrictional properties and also increases its strength. Hence, in orderto obtain a high degree of score resistance and adequate strength, it isnecessary that the alloy have a silicon content of at least 0.2%, andfor best results it is desirable to use at least 2% silicon. More thanapproximately 10% silicon should not be included in the alloy, however,because of casting difficulties; and the resultant bearing is toobrittle for practical applications if the silicon content is excessive.Furthermore, while an increased silicon content improves scoreresistance, the addition of silicon in amounts greater than about 5%provides only slight additional beneficial properties in this respect.Accordingly, best results are obtained for most purposes when thesilicon content is kept within the preferred range of approximately 2%to 5%.

In the present aluminum base bearing alloy, the bismuth greatly improvesthe frictional properties of the alloy and permits it to be used in theas-cast condition. However, an excessively high bismuth content impairsthe ductility of this alloy to too great an extent, and hence thebismuth should not be included in amounts greater than approximately 5%.A greater bismuth content results in an alloy which not only is toobrittle for most applications but which also presents casting problems.On the other hand, if the bismuth content is below 0.1%, the bearingalloy will have poor frictional properties and will be unsuitable foruse as a bearing. In view of these factors, it is generally preferred tomaintain the bismuth content between 0.2% and 1.5%. Even relatively highpercentages of bismuth within the aforementioned ranges, althoughlowering the ductility of the formed alloy some- What, are notparticularly detrimental in the manufacture of bearings since bearingsformed in accordance with the invention may be used in the as-castcondition and require very little ductility.

The chromium confers hardenability, high strength and machinability tothe resultant alloy. While the hardness Patented June 26, 1956 I may besubstantially reduced if the chromium content is too low, the additionof approximately 0.3% chromium is all that is necessary in order toobtain a completely satisfactory degree of hardness. Moreover, achromium content of only 0.2% chromium increases the hardness of thealloy considerably and makes it suitable for many bearing applications.I

The addition of chromium in amounts greater than about 0.8%, however,reduces the ductility of the resultant alloy to too great an extent, ahigh ductility being particularly necessary if the material is to beused as a wrought alloy. It is also not feasible to add more than 0.8%chromium because increasing the chromium content above this amountraises alloy costs by greatly in creasing the difficulty in casting andfabrication of the cast parts. Too high a temperature is required toplace and hold greater quantities of chromium in solution in the liquidstate, the chromium segregating out unless the temperature of the meltis raised excessively. The resultant formation of hard spots in thealloy prevents the obtaining of a uniform casting. A chromium contentbelow 0.1%, on the other hand, is insutficient to confer the necessaryhardness and strength to the alloy. Furthermore, the score resistance ofthe alloy is slightly improved as the chromium content is increased. Asa result of the above considerations, 1 have found that a chromiumcontent within the preferred range of approximately 0.2% to 0.35%provides excellent results in all respects.

An example of the above alloy which possesses the aforementioneddesirable characteristics to an outstanding degree, therefore, is oneconsisting of 4% silicon, 1% bismuth, 0.3% chromium, and the balance allaluminum and incidental impurities.

In order to obtain the high degree of resistance to pounding, such as isencountered in a bearing, it is preferable that the alloy have aphysical structure typified by the absence of continuous networks ofrelatively brittle eutectic mixtures. Unlike many of the aluminum basebearing alloys heretofore used, alloying the above composition does notinvolve problems due to vaporizing of any of the constituents, and closecontrols are not required because all of the elements used haverelatively low vapor pressures. The temperature of the melt may beraised as high as approximately 1600 F. before difiiculty is encounteredbecause of vaporization of bismuth, the element in the present alloyhaving the highest vapor pressure. Simple, conventional alloyingprocedures may be employed in the present instance, therefore, withintermediate alloys, such as aluminum-silicon and aluminum-chromiumalloys, being used to introduce the silicon and chromium. The bismuthmay then be added to the melt, which is subsequently stirred and cast inthe desired form, usually in metal or graphite molds.

Cast articles having a metallographic structure showing a continuousnetwork of segregated metal compounds may be improved as to strength andfatigue resistance by suitable heat treatment. For example, I have foundthat a solution treatment at a temperature between approximately 900 F.and 1050 F. for a period of eight to fifteen hours is particularlyeffective. Upon removing the alloy from the furnace following thesolution treatment, it is preferable to cool it immediately by quenchingin water. This treatment serves to increase the ductility of the alloyto an appreciable extent.

A precipitation treatment may thereafter be employed to provide thealloy with even greater hardness. This process is preferably carried outby heating the article for five to ten hours at a temperature in therange between approximately 300" F. and 400 F., a precipitationtreatment at 370 F. for eight hours being particularly satisfactory. Thealloy then may be again cooled, preferably in water, and suitablymachined. Such a heat treating process results in an article which isapproximately three or four times as hard as it was in the as-castcondition and whose fatigue strength is proportionally improved.

The specific gravity of the above-described alloy is about one-thirdthat of a tin-bronze bearing alloy, and has much greater resistance tofatigue or to cracking under the pounding action to which many types ofbearings are subjected. This property renders such an alloy suitable asa bearing for use under extreme conditions, tests on such bearingsindicating the relative absence of wear, either of the hearing or theshaft. In addition, the alloy appears to be resistant to corrosion byacid constituents of lubricating oils which attack many other bearingcompositions.

It is to be understood that, while the invention has been described bymeans of certain specific examples, the scope of the invention is not tobe limited thereby except as defined in the following claims.

I claim:

1. A hearing formed of an alloy consisting essentially of approximately0.2% to 10% silicon, 0.1% to 5% bismuth, 0.1% to 0.8% chromium, and thebalance substantially all aluminum.

2. A hearing formed of an alloy having high antifrietion properties andfatigue resistance in the as-cast condition, said alloy consisting ofapproximately 2% to 5% silicon, 0.3% to 2% bismuth, 0.1% to 0.8%chromium, and the balance aluminum plus incidental impurities.

3. A bearing as in claim 2 in which a surface thereof is provided with athin overlay of a metal of a class consisting of lead and lead basealloys.

4. A bearing characterized by high anti-friction properties andresistance to disintegration under impact and to attack by acidsdeveloped in lubricating oils, said bearing being formed of an alloyconsisting essentially of 2% to 5% silicon, 0.1% to 5% bismuth, 0.2% to0.35% chromium, iron not in excess of 0.5%, and the balancesubstantially all aluminum.

5. A hearing formed of an alloy consisting of 0.2% to 10% silicon, 0.1%to 5% bismuth, 0.1% to 0.8% chromium, and the balance aluminum.

6. A hearing formed of an alloy having high antifriction properties andfatigue resistance in the as-cast condition, said alloy consisting of 2%to 5% silicon, 0.3% to 2% bismuth, 0.1% to 0.8% chromium, and thebalance aluminum plus incidental impurities.

7. A hearing characterized by high score resistance formed of a heattreatable alloy consisting of 2% to 5% silicon, 0.3% to 2% bismuth, 0.2%to 0.35% chromium, iron not in excess of 0.5%, and the balance aluminum.

References Cited in the file of this patent UNITED STATES PATENTS1,940,922 Rainer Dec. 26, 1933 2,026,541 Kempf et al. Jan. 7, 19362,026,543 Kempf et al. Jan. 7, 1936 2,026,557 Kempf et a1. Jan. 7, 19362,076,578 Kempf et al. Apr. -13, 1937 2,238,399 Schluchter Apr. 15, 19412,325,071 Murray July 27, 1943 2,531,910 Hensel et al Nov. 28, 1950

1. A BEARING FORMED OF AN ALLOY CONSISTING ESSENTIALLY OF APPROXIMATELY0.2% TO 10% SILICON, 0.1% TO 5% BISMUTH, 0.1% TO 0.8% CHROMIUM, AND THEBALANCE SUBSTANTIALLY ALL ALUMINUM.